The heart of a computer is a magnetic disk drive which includes a rotating magnetic disk, a slider that has read and write heads, a suspension arm above the rotating disk and an actuator arm that swings the suspension arm to place the read and write heads over selected circular tracks on the rotating disk. The suspension arm biases the slider into contact with the surface of the disk when the disk is not rotating but, when the disk rotates, air is swirled by the rotating disk adjacent an air bearing surface (ABS) of the slider causing the slider to ride on an air bearing a slight distance from the surface of the rotating disk. When the slider rides on the air bearing the write and read heads are employed for writing magnetic impressions to and reading magnetic signal fields from the rotating disk. The read and write heads are connected to processing circuitry that operates according to a computer program to implement the writing and reading functions.
Magnetoresistive (MR) read sensors, commonly referred to as MR heads, are used in all high capacity disk drives. An MR sensor detects a magnetic field through the change in its resistance of as a function of the strength and direction of the magnetic flux being sensed by the MR layer. The standard type of MR sensor in disk drives manufactured today employs the tunneling magnetoresistive (TMR) effect, such that the resistance varies as a function of the spin-dependent quantum-mechanical tunneling transmission of the conduction electrons between two or more ferromagnetic layers separated by an insulating, non-magnetic tunneling barrier. The resistance of these sensors depends on the relative orientation of the magnetization of the different magnetic layers. For the TMR sensor, the electron flow provides a current perpendicular to-the-plane (CPP) of the magnetic layers. These devices are different in physical mechanism, material, and geometry than the current-in-plane giant magnetoresistive (CIP-GMR) sensor technology which they are presently replacing.
Not unlike their CIP-GMR predecessors, the resistance for TMR sensors depends primarily on the relative magnetization of only two layers of ferromagnetic material (e.g., CoFe), which in the TMR case are separated by a very thin (˜1 nm) insulating tunnel barrier layer (e.g., MgO). In a “simple” TMRsensor, one of the ferromagnetic layers, referred to as the reference layer (or pinned layer), has its magnetization typically pinned by exchange coupling with an antiferromagnetic (e.g., IrMn) layer. The pinning field generated by the antiferromagnetic layer should be sufficiently large to ensure that the magnetization direction of the reference layer remains fixed during the application of external fields (e.g., fields from bits recorded on the disk). The magnetization of the other ferromagnetic layer, referred to as the free layer, however, is not fixed and is free to rotate in response to the field from the recorded magnetic medium (the signal field). U.S. Pat. No. 5,206,590 granted to Dieny et al., incorporated herein by reference, discloses a “simple” CIP-GMR sensor operating on the basis of the GMR effect.
Almost universally employed in present day sensors ((either TMR or CIP-GMR) is the use is the use of antiparallel (AP)-pinning. In such AP-pinned sensors, the reference layer is a laminated structure of two ferromagnetic layers separated by a non-magnetic AP-coupling layer such that the magnetizations of the two ferromagnetic layers are strongly coupled together antiferromagnetically in an antiparallel orientation. The first ferromagnetic layer, referred to as the pinned layer, has its magnetization pinned/fixed in orientation by direct exchange coupling to an AFM layer. The second ferromagnetic layer serves as the reference layer in determining the resistance of the device, is strongly AP-coupled to the pinned layer, and by effect is also fixed in orientation. The cancellation of magnetic moment and demagnetizing fields of the AP-aligned pinned and reference layers greatly improves the stability of the reference layer relative to that obtained for the simple SV sensor of FIG. 1A.
Referring to FIG. 1B, an AP-Pinned TMR sensor 200 comprises a free layer 210 separated from a laminated AP-pinned layer structure 220 by a nonmagnetic, electrically-conducting spacer layer 215. The magnetization of the laminated AP-pinned layer structure 220 is fixed by an AFM layer 230. The laminated AP-pinned layer structure 220 comprises a first ferromagnetic (pinned) layer 226 and a second ferromagnetic (reference) layer 222 separated by an antiparallel coupling layer (APC) 224 of nonmagnetic material. The two ferromagnetic layers 226, 222 (FM1 and FM2) in the laminated AP-pinned layer structure 220 have their magnetization directions oriented antiparallel, as indicated by the arrows 227, 223 (arrows pointing out of and into the plane of the paper respectively).
For TMR sensors, the conductance G of the TMR sensor (more so the resistance R=1/G), is believed to be a linear function of cos(θ), where θ is the angle between the (in-plane) magnetization vectors of the reference and free layer structures. Specifically, θ≡θf−θr, where θf is the angle of (in-plane) magnetization of the free layer and θr represents the angle of (in-plane) magnetization of the reference layer. The sensitivity of the sensor can be quantified by its magnetoconductance coefficient ΔG/Gmin, where ΔG=Gmax−Gmin is the maximum change in the conductance of the sensor. However, it is much more common, today (and historically) to characterize TMR sensors by the TMR ratio ΔR/Rmin, where ΔR=Rmax−Rmin the maximum change in the resistance of the sensor. It is virtually always the case in TMR sensors as practiced in the art that Rmax=R(θ=180°)=1/Gmin and Rmin=R(θ=0)=1/Gmax. The TMR ratio ΔR/Rmin, is mathematically identical in magnitude to, ΔG/Gmin.
In operation, the sensor is subjected to positive and negative magnetic signal fields Hsig from a moving magnetic disk. These positive and negative signal fields are typically equal in magnitude, and oriented orthogonal to the plane of the disk (or ABS plane). In addition to maximizing the magnitude of the readback signal from the TMR sensor, it is also desirable that positive and negative readback signals are equal as well.
It is well known magnetically that the rotation of the free layer magnetization angle θf in response to magnetic signal fields from the disk, is such that sin(θf) will vary approximately linearly (to first order) with the amplitude of the signal field Hsig. This is particularly true if θfb=0 is approximately the (quiescent) bias-point orientation of the free layer in the absence of signal fields, in which case the sensitivity d(sin(θf)/dHsig is also generally maximized. It follows that θfb≅0° will be a near-optimal bias point configuration with respect to the free layer:
However, consideration of the optimized (pinned) angle θr for the reference layer in a TMR sensor can be different than that of θr≅±90° that was historically practiced in the art for CIP-GMR sensors, and which often continues for current art TMR read sensors. In particular, if it is the conductance G that varies linearly with cos(θ), i.e.,
      G    =                  G        min            +                        1          2                ⁢                  (                      1            +                          cos              ⁢                                                          ⁢              θ                                )                ⁢        Δ        ⁢                                  ⁢        G              ,it follows that
            R      b        =                  R        max                    1        +                              (                          Δ              ⁢                                                          ⁢                              R                /                                  R                  min                                                      )                    ⁢                      1            2                    ⁢                      (                          1              +                              cos                ⁢                                                                  ⁢                                  θ                  r                                                      )                                                                  ⅆ          R                          ⅆ                      H            sig                                      =                                        R            max                    ⁢                                                sin              ⁢                                                          ⁢                              θ                r                                                                                      [                          1              +                                                (                                      Δ                    ⁢                                                                                  ⁢                                          R                      /                                              R                        min                                                                              )                                ⁢                                  1                  2                                ⁢                                  (                                      1                    +                                          cos                      ⁢                                                                                          ⁢                                              θ                        r                                                                              )                                                      ]                    2                    ⁢                                                          ⅆ              sin                        ⁢                                                  ⁢                          θ              r                                            ⅆ                          H              sig                                                  assuming the aforementioned optimum free layer bias-point angle θfb≅0°
In the case where ΔR/Rmin<<1, it is readily deduced that the optimum reference layer point is θr≅±90°, as this both maximizes sensitivity |dR/dHsig| and puts the bias resistance point Rb≅½(Rmin+Rmax) at the midpoint for maximizing dynamic range and minimizing asymmetry of sensor response to opposite polarities of Hsig. This circumstance applied to traditional CIP-GMR sensors, as well as older TMR sensors such as those made using Alumina (Al2O3) tunneling barriers. However, for state-of-the-art TMR read sensors (e.g., CoFeB magnetic layers with MgO tunnel barriers) where ΔR/Rmin≈1, it follows from the above equation that both |dR/dHsig| and Rb will be better optimized when cos θr≅cos θ<0 is negative, corresponding to an obtuse bias angle |θb=θfb−θr|>90°. The actual optimum point, by these criteria, depends on how large the TMR ratio ΔR/Rmin is for a given sensor.
The present invention addresses several different ways to achieve this non-orthogonal bias point